1. Field of the Invention
The present invention relates to a data converting apparatus and a data converting method, which receive an audio signal and output the audio signal at a changed reproducing speed.
2. Description of the Related Art
A technique is in practical use, which reproduces an audio signal while changing a reproducing time of the audio signal without changing its pitch. The technique is used for applying sound effects in so-called KARAOKE, and in a fast-forward reproduction and/or in a slow-speed reproduction of CD, video tapes, and DVD. The technique is roughly divided into two groups: one is based on operation on a time axis and other is based on operation on a frequency axis. As the typical example of the former technique, OLA (Over and Add) method and its developed method PSOLA (Pitch Synchronous OLA) method are known, and as the typical example of the latter technique, the phase vocoder is known.
The OLA method is described, for instance, in Japanese Patent NO.2005-266571 A. In the OLA method, audio data is divided into plural units of frames, and these frames are subjected to a window function. The windowed frames are successively disposed with their portions overlapped each other and data values of the overlapped portions are added and output. By changing lengths of the overlapped portions, the audio data is extended and/or contracted. The OLA method is simple in processing, and can be realized by a simple configuration. But disconnection can occur around at connecting portions of frames including overlapping portions of the frames, causing noises.
The PSOLA method is disclosed, for example, in Japanese Patent NO. Hei10-124082 A. The PSOLA method extracts a pitch of audio data and sets a frame size of the audio data to be divided based on the extracted pitch, thereby reducing noises caused at connecting portions of the frames. But the PSOLA method has a drawback that the PSOLA method can be used only for audio data having a pitch that can be extracted, in other words, the PSOLA method can be used only for audio data of musical instruments of a single tone.
The technique based on the operation on the frequency axis is disclosed by U.S. Pat. No. 7,672,835. In the technique based, on the operation of the frequency axis, such as phase vocoders, the audio data is subjected to Fourier transform to operate the phase on the frequency axis, thereby computing the inverse Fourier transform to obtain data on the time axis again. In this way, the audio data is extended and/or contracted. This technique can obtain better results than OLA method and PSOLA method, but includes a problem that requires complex arithmetic processing such as Fourier transform and complex number operation.